Pipe Flanges: Key Considerations for Metal Hoses and Expansion ...

By: Frank Caprio

While corrugated metal hoses and expansion joints can accommodate many different end fitting types, pipe flanges are one of the most common connections specified by customers. However, we find there is a good deal of confusion in our industry as to the proper use of pipe flanges on metal hoses and expansion joints. Here are a few things to keep in mind when helping customers with flanged assemblies.

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Defining Pipe Flanges

Pipe flanges allow for easy installation of the component into a piping system. Proper flange usage avoids many potential problems associated with threaded connections (space limitations, thread allowances, numerous thread standards, thread sealing challenges, etc.). Flanges must be manufactured in accordance with well-defined specifications, which are then referenced in piping codes that are used in various industries. Industry relies on these codes and standards to ensure the safety and consistency of a properly designed piping system.

Fixed Flanges

Pipe flanges can be attached to flexible connectors (metal hoses and expansion joints) in a number of ways, and it is important to recognize the advantages of each attachment style. First, a flange may be directly welded to the end of the hose or expansion joint, which is referred to as a “fixed” flange. This is the most basic method of attachment. It is important to note that a flange welded directly to a flexible metal hose may require a section of pipe (a “pipe spacer”) to be welded between the hose and the flange, in order for the assembly to conform to various piping code requirements.

If a hose or expansion joint has fixed flanges on both ends, it is critical that these flanges align properly with the mating connections. Otherwise, the connector may be twisted in an attempt to align the bolt holes during installation, creating torsional stresses that can damage the connector. The NAHAD Hose Safety Institute lists recommendations for proper bolt-hole alignment, which follows sound design practices and conforms to ASME’s Pressure Piping Code B31. If non-standard bolt-hole orientations are present, a good way to ensure proper alignment is to wait until the hose or expansion joint has been installed before welding the mating flange onto the adjoining piping system. This will ensure that the flange on the pipe will properly align with the flange on the hose end.

Floating Flanges

Another way to avoid flange misalignment is by installing a flange that rotates on the end of the flexible connector. These are referred to as “floating” flanges and come in various configurations. Floating flanges rotate without twisting the flexible connector, allowing the installer to align the bolt holes to the flange on the mating piping system. Once these floating flanges are in place and bolted to the mating flange, they no longer swivel. This means they cannot accommodate rotational movements after installation. If a rotating movement is required during operation, a properly designed swivel joint should be used to prevent twisting the flexible connector. For flexible metal hoses, flanges are installed onto a stub end, which is then welded to the hose. The face of the stub end becomes the sealing surface for a floating flange. Some stub ends are made by flaring a piece of pipe on one end; others are made by machining the stub end out of solid material. They can be made in various alloys and pipe schedules, and have different configurations that accommodate different flange types.

C-Stub with Floating Flange

Vanstone Flanges

While metal expansion joints may also contain floating flanges on stub ends, there is an alternative that provides a floating flange with a much more compact profile: Vanstone flanges. Vanstone flanges are attached directly to a metal bellows by slipping the flange over the unconvoluted skirt of the bellows (also called the “cuff” or “tangent”), and then flaring out the skirt to form a sealing surface over the face of the flange.

Vanstone flange connections take up much less length than stub ends, while still allowing the flange to rotate during installation. Because no welding is required for a Vanstone connection, it is a very economical means of flange attachment. However, the Vanstone connection may not be suitable for high-pressure applications, and require a special gasket, so due diligence is needed before these should be recommended.

Vanstone Flange Diagram

Not only do floating flanges compensate for bolt-hole misalignment in a piping system, they also isolate the flange from the media being conveyed. This means that a flange of a less expensive alloy may be allowed, as it does not come into direct contact with the material being conveyed.

Trust the Experts

There are many different flange specifications out there, and knowing which one to use can be confusing. It is not merely a “good idea” for any piping design to be reviewed by a qualified piping engineer or designer; failure to do so can lead to a catastrophic failure of the system. Once the proper flange style has been identified, the pressure class, alloy, method of attachment, gasket selection, nut/bolt specifications, and other factors must then be considered. This will ensure the flanged connections are safe and appropriate for the intended service.

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In the realm of fluid and gas transfer, selecting the appropriate hose is crucial for ensuring efficiency, safety, and longevity of the system. Two commonly used types are flexible metal hose and braided hoses. While they might appear similar, their characteristics and applications differ significantly. This comprehensive guide delves into the intricacies of both, helping you make an informed decision for your specific needs.

Understanding Flexible Metal Hoses

Definition and Structure

Flexible metal hoses are engineered to transport fluids and gases under varying pressure and temperature conditions. They are typically constructed from materials like stainless steel, bronze, or Monel, which impart durability and resistance to corrosion. The core component is a corrugated metal tube, designed to provide flexibility while maintaining structural integrity. To enhance pressure resistance, these hoses often feature an external braiding of stainless steel wire.

Key Characteristics

• Flexibility: The corrugated design allows for significant flexibility, accommodating movements, vibrations, and thermal expansions without compromising the hose’s integrity.
• Durability: Constructed from robust metals, these hoses exhibit high resistance to wear, corrosion, and extreme temperatures, making them suitable for harsh environments.
• Pressure Resistance: The external braiding not only protects the hose but also enhances its ability to withstand higher internal pressures.

Also Read: Camlock Couplings: 8 Important Things You Need to Know

Common Applications

Flexible metal hoses are utilized across various industries due to their versatility:

• Chemical Processing: Ideal for transporting corrosive chemicals and acids, thanks to their corrosion resistance.
• Food and Beverage: Used for transferring food products and liquids, as they are hygienic and easy to clean.
• Automotive Industry: Employed in exhaust systems as vibration decouplers, handling high temperatures and movements.
• Aerospace: Suitable for applications requiring flexibility under extreme conditions, such as in aircraft fuel and hydraulic systems.

Exploring Braided Hoses

Definition and Structure

Braided hoses consist of an inner flexible tube, typically made from materials like rubber or Teflon (PTFE), reinforced with an external layer of braided metal or synthetic fibers. The braiding provides structural support, enhancing the hose’s strength and pressure-handling capabilities.

Key Characteristics

• Enhanced Pressure Capacity: The braided reinforcement allows these hoses to handle higher pressures compared to non-reinforced hoses.
• Flexibility: While offering substantial strength, braided hoses maintain flexibility, making them suitable for dynamic applications.
• Versatility: Depending on the materials used for the inner tube and braiding, these hoses can be tailored for specific applications, including chemical resistance or high-temperature tolerance.

Common Applications

Braided hoses are prevalent in various sectors:

• Automotive and Transportation: Commonly used in brake systems to provide flexibility and strength, ensuring efficient braking performance.
• Industrial Machinery: Utilized in hydraulic and pneumatic systems where high-pressure fluid transfer is required.
• Residential Plumbing: Employed in household applications such as connecting faucets and toilets, owing to their flexibility and ease of installation.

Comparative Analysis: Flexible Metal Hose vs. Braided Hose

When deciding between flexible metal hoses and braided hoses, several factors come into play:

Pressure Handling

• Flexible Metal Hoses: Withstand high pressures, especially when reinforced with external braiding.
• Braided Hoses: Pressure capacity varies based on the materials used but generally suitable for medium to high-pressure applications.

Recommended Reading: Camlock Coupling Types: Choosing the Right Quick-Connect Coupling for Air, Water, Oil, and High-Pressure Applications

Temperature Resistance

• Flexible Metal Hoses: Excel in high-temperature environments, capable of handling extreme heat without degradation.
• Braided Hoses: Temperature tolerance depends on the inner tube material; for instance, PTFE-lined hoses can handle higher temperatures than rubber-lined ones.

Flexibility and Bend Radius

• Flexible Metal Hoses: Offer good flexibility but have a larger bend radius compared to braided hoses.
• Braided Hoses: Typically more flexible with a tighter bend radius, making them ideal for confined spaces.

Durability and Longevity

• Flexible Metal Hoses: Highly durable, resistant to corrosion, and have a long service life, even in harsh conditions.
• Braided Hoses: Durability varies with material; metal-braided hoses offer good durability, while synthetic-braided ones may have a shorter lifespan.

Cost Considerations

• Flexible Metal Hoses: Generally more expensive due to material and construction complexity.
• Braided Hoses: Often more cost-effective, especially those with synthetic braiding.

Making the Right Choice for Your Application

To determine the most suitable hose for your application, consider the following aspects:

• Operating Conditions: Assess the pressure, temperature, and environmental factors the hose will be exposed to.
• Flexibility Requirements: Identify the necessary flexibility and bend radius for your system layout.
• Chemical Compatibility: Ensure the hose materials are compatible with the fluids or gases being transported.
• Regulatory Standards: Verify compliance with industry standards and regulations pertinent to your application.
• Budget Constraints: Balance the initial investment with the expected lifespan and maintenance costs.

Selecting between flexible metal hoses and braided hoses hinges on a thorough understanding of your application’s specific demands. While flexible metal hoses offer superior durability and temperature resistance, braided hoses provide versatility and cost-effectiveness. By meticulously evaluating the operational requirements and environmental conditions, you can choose a hose that ensures safety, efficiency, and longevity in your fluid or gas transfer system.

For more detailed information on flexible metal hoses, you can refer to the Wikipedia page on metal hoses.

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